Something that's always worked out well for me is to go through the websites of all of the major theatre conferences- SETC, UPTA, USITT, etc, and find the list of companies in attendance. Sometimes it just lists who is going to be there, sometimes it's as detailed as what positions they are interviewing for at the conference. Make a list of companies you're interested in and research them online, then send them your application- you don't get a face-to-face interview like you would at the conference, but most of them accept email/hard copy applications.
Also, it's pretty early for the hiring process to start for most summer companies, so a lot of them may not have updated hiring/application information on their websites yet.